In fact all issues come from the fact JBoss load the persistence.xml too early. I've tested to update the <provider> tag with a little patch by using "org.hibernate.ejb.HibernatePersistenceMyImpl" (this class has to start with "org.hibernate.ejb.HibernatePersistence" to be considered by Hibernate configuration, but hibernate dos not use the CL of this provided class, but the one of "PersistenceUnitInfo" (see Ejb3Configuration#configure(PersistenceUnitInfo, Map). One solution could consist to wrap the "PersistenceUnitInfo" with a custom proxy class to force the CL....
Anyway, to be fully compliant with JBoss (and some other JEE servers) I've switch to this solution : http://fabdouglas.blogspot.com/2011/01/how-to-run-spring-3-jpa-application-to.html